Fundamentals in Organic Light Emitting Diodes

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An organic light-emitting diode (OLED) is made of a stack of organic semiconductor layers sandwiched between a transparent conducting anode and a reflective metal cathode. Under an external electric bias between the anode and the cathode, holes, or positive charges, from the anode and electrons from the cathode are forced into the organic semiconductor layers to form excitons (tightly bound hole-electron pairs). The annihilation of an exciton may release its energy radiatively to emit light or non-radiatively to generate heat. Some of the emitted light may be trapped inside the device due to optical waveguide and other optical effects. The percentage of the emitted light coming out of an OLED (i.e. optical coupling factor) is related to the orientation of organic molecules in these thin-films. The device’s external quantum efficiency is determined by these three processes: electron-hole balance factor, excitonics, and optical coupling. For electron-hole balance factor, the research will focus on understanding energy barriers at electrode-organic interfaces and organic hetero-junctions by using X-ray photoelectron spectroscopy (XPS) and ultra-violet photoelectron spectroscopy (UPS). For excitonics, time-resolved PL and EL will be used to quantify several competing excitonic processes: migration and radiative recombination, non-radiative recombination, and exciton energy transfer from one emitter molecule to another. For optical coupling, the research objective will be understanding of materials science governing emitter molecular orientations in guest-host systems.